Hydrolytic Decomposition of S-Aroylthiooximes: Effect of pH and N-Arylidene Substitution on Reaction Rate

The hydrolytic decomposition of four peptides containing S-aroylthiooximes (SATOs) with variable N-arylidene substituents was investigated in 10 aqueous buffer solutions at pH values ranging from 6.0 to 10.9. UV–vis spectroscopy was employed to study the reaction kinetics, which revealed V-shaped pH–rate profiles for all peptides with a minimum near pH 8, suggesting a change from an acid-catalyzed to a base-activated reaction. Hammett plots showed positive ρ values above pH 8 and negative ρ values below pH 8, providing further evidence for a mechanism change. Based on these data, along with mass spectral evidence, we propose specific acid catalysis under mildly acidic and neutral conditions and multiple base-promoted decomposition reactions under mildly basic conditions.